Short-carbon-fiber-reinforced thermoplastic resin (hereinafter referred to as “CFRTP”) produced using a thermoplastic resin as a matrix resin, has drawn attention as a high-performance engineering material, and the demand therefor is increasing rapidly. This CFRTP can be produced by injection molding and accordingly is high in productivity. Moreover, the CFRTP, as compared with conventional non-reinforced thermoplastic resins or short-glass-fiber-reinforced thermoplastic resins, is superior in mechanical properties, sliding property, electrical properties, dimensional stability, etc.
As the method for producing the CFRTP, there are ordinarily the following methods.    (1) First, there is fed, into an extruder, a carbon-fiber filament bundle cut into 3 to 10 mm and bundled with a sizing agent (a so-called carbon-fiber chopped strand) or a so-called milled carbon fiber ground into 1 mm or shorter, together with pellets or a powder of thermoplastic resin; and they are melt-kneaded therein to obtain pellets. Then, the pellets are made into a CFRTP using an injection molding machine or an extrusion molding machine.    (2) A carbon-fiber chopped strand is fed into an extrusion molding machine together with pellets or a powder of thermoplastic resin to directly produce a CFRTP.
Meanwhile, as the method for feeding a carbon-fiber chopped strand and a thermoplastic resin into an extruder to produce pellets, the following methods are employed mainly.    (1) A method of dry-blending a carbon-fiber chopped strand and a thermoplastic resin and feeding the resulting blend into an extruder (a dry-blending method).    (2) A method of feeding a thermoplastic resin into the rear end side (in extrusion direction) of extruder and feeding a carbon-fiber chopped strand to the intermediate portion (in extrusion direction) of extruder wherein the fed thermoplastic resin is in a molten state (a side-feeding method).
As well known, the properties of CFRTP are related to the fiber length of the carbon fiber of CFRTP. When there is used a milled fiber of extremely short fiber length, the fiber length in the molded CFRTP is extremely short; accordingly, the properties of this CFRTP are inferior to those of CFRTP using a carbon-fiber chopped strand.
In order to keep long the fiber length of CFRTP, there is a case that a CFRTP is produced using a long fiber pellet having a fiber length same as the cut length of carbon-fiber chopped strand. In this case, the fiber direction in the CFRTP obtained is difficult to control. Therefore, this method of CFRTP production using a long fiber pellet is disadvantageous for production of an inexpensive CFRTP which needs to be mass-produced. For the above reasons, carbon-fiber chopped strand is generally used in production of CFRTP.
In production of CFRTP, when the carbon-fiber chopped strand used is low in flowability, there is a problem that stable feeding of carbon-fiber chopped strand into extruder is difficult.
In the dry-blending, when the carbon-fiber chopped strand used is low in flowability, there is a problem that the carbon-fiber chopped strand is difficult to flow down in the hopper of extruder or injection molding machine. As a result, the feeding of the carbon-fiber chopped strand in a given amount from the meter provided at the bottom of hopper, to the extrusion screw of extruder becomes unstable. For the above reason, it is difficult to obtain a CFRTP of uniform composition stably and, moreover, the production efficiency is low.
Meanwhile, in the side-feeding method as well, when the carbon-fiber chopped strand used is low in flowability, the carbon-fiber chopped strand is not fed stably into the extrusion screw of extruder and, in an extreme case, the feeding itself of the carbon-fiber chopped strand may be impossible.
For these reasons, a carbon-fiber chopped strand used industrially in a large amount is required to have high flowability. In order to respond to this requirement, it is conducted to add a sizing agent of high bundling ability to a carbon-fiber chopped strand or add a sizing agent in a large amount. It is also conducted to add another sizing agent to a carbon-fiber chopped strand obtained by cutting a strand, to mold the carbon-fiber chopped strand into a rice grain shape.
However, in producing a CFRTP by mixing the above-mentioned chopped strand containing a large amount of a sizing agent, with a heat-resistant thermoplastic resin of high processing temperature, a gas is generated by the thermal decomposition of the sizing agent. This gas allows the obtained CFRTP to have poor appearance or low weld strength (see, for example, Patent Literatures 1 and 2). Further, the thermal decomposition of the sizing agent tends to cause reduction in properties of CFRTP.
Further, in melt-kneading a chopped strand with a thermoplastic resin in an extruder, there is a case that the sizing agent added in a large amount to the carbon fiber of chopped strand reduces the dispersibility of the carbon fiber. In this case, the dispersibility of carbon fiber in the pellet obtained is insufficient. When a CFRTP is produced using this pellet, there are present, in the CFRTP produced, fiber bundles not dispersed sufficiently. The fiber bundles become a center of stress concentration, reducing the mechanical properties (particularly, tensile strength) of the CFRTP.
Meanwhile, in order to produce a chopped strand in a large amount, it is effective to increase the number of single filaments constituting the chopped strand. Chopped strands composed of 30,000 or more single filaments are known. Such a chopped strand has a flat shape. The flat shape makes easy the single-filament-state dispersion of carbon fiber in CFRTP, whereby the assembling of carbon fibers in bundle shape can be avoided.
However, the chopped strand of flat shape has a large surface area, resulting in a large contact area between chopped strands. Consequently, the chopped strand has low flowability and, in feeding the chopped strand into an extruder, there arises poor feeding into meter or extruder.
Further, the above poor feeding makes long the residence time of chopped strand in extruder. In this case, the chopped strand undergoes larger shear by the screw of extruder and the breakage of carbon fiber takes place. As a result, the fiber length becomes short, resulting in reduction in mechanical properties of the CFRTP obtained.
As described above, it has been difficult to allow a carbon-fiber chopped strand to have high flowability without reducing the dispersibility of carbon fiber and the properties of the CFRTP obtained and thereby to feed the chopped strand in a large amount in a stable state from the hopper of extruder to the extrusion screw of extruder.
Patent Literature 1: JP-A-2003-165849
Patent Literature 2: JP-A-2004-149725